The transgenic Mouse/ES Cell Chimeras Facility provides four basic services: production of transgenic (Tg) mice by zygote injection, production of embryonic stem (ES) cell chimeric animals, rederivation of mouse strains and isolation of gene-targeted ES cells following homologous recombination. For production of transgenic mice, users supply DNA constructs suitable for pro-nuclear injection into fertilized mouse eggs. Transgenic mice are produced in outbred Swiss Webster mice, inbred FVB/N mice or mice of defined genotype. Potential founder animals are provided to the research investigator at weaning for DNA analysis by Southern blotting for genotyping. ES cell chimeras as produced by morulae aggregation using the Swiss Webster strain as a host or by injection of ES cells into C57BL/6 blastocysts. Chimeras are provided to the research investigator at weaning for test breeding to determine germline transmission of the mutation. Aggregation chimeras between two strains of mice can also be made through the TgESC facility. A recent service being offered involved the new technique of making chimeras with tetraploid morulae to produce embryos that are 100% ES cell-derived. To enable investigators to house pathogen-free animals in the SPF animal quarters in Skirball, rederivation of mouse strains is also offered. For isolation of gene-targeted ES cells, users of the resource supply DNA targeting constructs prepared in consultation with the facility and a confirmed genotyping assay. Constructs are electroporated into ES cells prepared in the facility, and isolated clones are scored using the genotyping assay. Positive homologous recombinants are transferred to the chimera unit for transmission of the mutant allele into mice. In addition, the facility provides researchers with a wide array of services related to the production of transgenic and knockout ES cells and chimeric mice such as supply of superovulated egg donors, pseudopregnant females, training in mouse husbandry and consultation in transgenic/ES cell approaches and vector design.